Calling revlog.addgroup with an empty changegroup now raises RevlogError.

The empty changegroup can be caused by remote servers dying soon after
findincoming, and further code in pull assumes (correctly) that there are
new changesets.

mercurial/revlog.py

@@ -743,7 +743,7 b' class revlog(object):'
         #track the base of the current delta log
         r = self.count()
         t = r - 1
-        node = nullid
+        node = None
 
         base = prev = -1
         start = end = measure = 0
@@ -816,6 +816,8 b' class revlog(object):'
 
         dfh.close()
         ifh.close()
+        if node is None:
+            raise RevlogError(_("group to be added is empty"))
         return node
 
     def strip(self, rev, minlink):